This invention relates to an optical network operable as a subcarrier-frequency multiplexed system and to an analog repeater node for use in the optical network.
Optical communication has hitherto been developed on a basis of digital synchronization hierarchy (SDH). Optical networks have therefore been studied on the basis of the SDH. A salient feature of the SDH is to provide for unified dealing with all information, such as telephone information and data information. In order to unify such services, developments are in progress towards BISDN's (broadband integrated services digital networks). As a consequence, it is usual to design an optical network with attention directed to digital multiplexing based on the SDH.
In the meanwhile, use of the optical communication has widely spread to various applications. Typical are applications in cable television and in mobile communications. In these applications, transmission signals may be analog signals. Such signals are transmitted by frequency multiplexing into frequency multiplexed signals. In the optical networks, it is often desirable to propagate these signals through optical fibers as they are. It is therefore advantageous to use subcarrier-frequency multiplexed optical communication wherein the frequency multiplexed signals are propagated as they stand.
This subcarrier-frequency multiplexed optical communication does not necessarily well match with conventional SDH. It is consequently important with progresses in applications of the subcarrier-frequency multiplexed optical communication to establish an optical network on a large scale on the basis of the subcarrier-frequency multiplexing.
An excellent optical network is disclosed in an article contributed in the Japanese language by Takehiro Shiozawa and Makoto Shibutani to the transactions of an Electronics, Information, and Communication System Study Group of the Institute of Electronics, Information and Communication Engineers, CS 92-6 (May 28, 1992), pages 33 to 38, under the title of "U-FDMA/D-TDM Optical Fiber Multiaccess Subscriber Transmission System". A similar optical network is revealed by Takahiro Shiozawa and two others in the IEEE International Conference on Communications, Jun. 14-18, 1992, pages 304.7.1 (0105) to 304.7.5 (0109), under the title of "Upstream-FDMA/down- stream-TDM Optical Fiber Multiaccess Network".
Such a Shiozawa et al optical network comprises a plurality of terminator nodes connected to a central office through at least one star coupler. Each terminator node comprises several network terminals.
For upward or upstream transmission towards the central office, the network terminals of each terminator node are assigned with discrete signal frequencies. In each terminator node, transmission signals of the discrete signal frequencies are multiplexed into a frequency multiplexed signal. Each terminator node is assigned with an individual optical wavelength as an assigned wavelength and sends to the star coupler the frequency multiplexed signal on an optical signal of the assigned wavelength.
For downward or downstream transmission from the central office, the network terminals of the Shiozawa et al optical network are assigned with respective time slots to enable use of a time division multiplexing (TDM) technique. Although the TDM technique is preferred according to Shiozawa et al, it is possible to use a wavelength division multiplexing (WDM) technique also for the downstream transmission.
On establishing an optical network based on subcarrier-frequency multiplexing, each terminator node can independently transmit the transmission signals on optical signals if the terminator nodes are assigned signal frequency bands which do not interfere with one another. Even in this event, a problem may arise from optical interference noise when such optical signals are transmitted simultaneously from a plurality of terminator nodes. This problem of optical beat interference noise is pointed out by Shiozawa et al and is removed by Shiozawa et al with use of different optical wavelengths in the terminator nodes, respectively. It is, however, impossible to establish the optical network on a large scale if such discrete frequencies and different optical wavelengths must be used individually in the terminator nodes of the optical network.